Batteries have been widely used in electric motor systems, such as electric automobiles. These batteries must be charged or recharged periodically after being depleted. A battery charger is usually used to serve the charging or recharging purpose. Typically, the battery charger is connected to an AC source which supplies a rectified charging voltage, in the form of positive alternations of juxtaposed sine waves varying between a zero voltage and a crest voltage greater than that of the battery.
During each voltage alternation, the battery charger can only provide power when it has a greater voltage than that of the battery, since the voltage of the battery is opposite to that supplied by the battery charger. When the voltage of the battery charger is below the battery voltage, the battery charger is unable to overcome the battery voltage and will hence not supply any power. Therefore, each half-cycle of the source voltage, will pass from an active state, in which the source will feed the battery charger to thus supply power, to a blocked state, which practically has no consumption.
These abrupt changes of load from the source can induce harmonics, which would become more intense if the battery charger is designed to be more powerful to limit the load time. Such harmonics can cause interference with equipments, such as television sets, of other users of the source. Therefore, the maximum level of the battery charges is strictly regulated.
In order to eliminate these harmonics, it is therefore necessary for the battery charger to supply power, even at a low rectified voltage. In other words, the battery charger simulates a passive load when voltage is lacking, in place of the battery. More precisely, a constant passive load is simulated, that is to say, the input current of the battery charger must be proportional to the input voltage. Preferably, to the input current is effectively in phase with the input voltage in order to simulate, on the source side, a purely resistive load and therefore avoid calling on current out of phase, which is a source of losses on the line.
In order to maintain the flow inside the battery charger when the battery charger has a voltage lower than that of the battery, a comparator is provided. The comparator compares the rectified voltage that it produces to that of the threshold of the battery, thus in some manner disconnecting the battery intermittently from the battery charger in the phase where the battery voltage is too low and causing the flow to only go to an inductor which, therefore, does not have a threshold voltage. This inductor is installed in series on one of the two output terminals of the charger and can be looped to the other terminal by the closing of a switch. The battery is connected to the terminals of the switch through a one-way diode to prevent the battery from being short-circuited by the switch. The closed switch therefore provides a tap for the current feeding the inductor. Regulation through cut-off at the level of the switch makes it possible to regulate the current, which is measured so that it will follow the sine wave form of the incoming voltage.
When the switch is open, the inductor provides an over-voltage which makes it possible to overcome the voltage of the battery and thus discharge the voltage stored by the inductor to restart a new cycle. This operation, which makes it possible definitively to raise the voltage of the power supplied at low voltage, is called "boost".
During the phase of a high rectified voltage, the charging current can flow directly from the charger into the battery, since it supplies a voltage exceeding that of the battery voltage.
However, the battery is then equivalent to a short circuit, since it has a fixed voltage regardless of current. The charging current is therefore controlled by a series cut-off switch, of which the percentage of time of closing regulates the current. This operation mode is called "buck".
Such a battery charger is complex, because it includes two assemblies with current regulation. Further, it is necessary to monitor the incoming voltage to activate one while de-activating the other. In addition, since each assembly has its own regulating circuits, the switch from one mode of operation to the other causes an abrupt transition in the regulation which has repercussions on the source.
Even though it is possible to provide a single installation which simultaneously permits the two operations, the inductor must then be over-dimensioned because all power passes through it.